1. Field of the Art
This invention relates to a reflecting mirror capable of reflecting light beams of discrete wavelengths, and an optical pickup incorporating the reflecting mirror.
2. Prior Art
Generally, an optical pickup is constituted by a light source, polarized beam splitter, λ/4 wave plate, objective lens, APC (Auto Power Control and a photodetector. A light beam from a light source is fed to a polarized beam splitter to separate p-polarized light and s-polarized light. One of separated p- and s-polarized light beams is shed on an optical disc while the other one is shed on an APC for detection of signals. The light beam to be shed on an optical disc is converted from linearly polarized light to circularly polarized light by a λ/4 wave plate and reflected off the optical disc, and further converted from circularly polarized light to linearly polarized light by the λ/4 wave plate before entering the polarized beam splitter again. Since the direction of polarization is shifted 90 degrees at the time of re-entering the polarized beam splitter, the incident light beam is either transmitted through or reflected off and shed on a signal detector. Generally, in an optical pickup of this sort, a light beam is projected from a light source along a light path parallel with the surface of an optical disc, and the parallel light path is turned toward the optical disc by the use of a reflecting mirror (turning mirror).
Optical pickups employs a laser beam and thus a light source which outputs a laser beam. On the other hand, as the reflecting mirror mentioned above, there may be employed a reflecting mirror having a metal film deposited on a surface of a substrate or a reflecting mirror having a dielectric multi-layer film coating formed by alternately laminating a high refractivity layer and a low refractivity layer one after another. Because a metal film is susceptible to corrosion, oxidation and bruises or other damages, and because of the ability of efficiently reflecting off a specific wavelength, it has been the general practice for an optical pickup to employ a reflecting mirror with a dielectric multi-layer coating.
In the case of a reflecting mirror with a dielectric coating, generally TiO2 and SiO2 are used for the high and low refractivity layers, respectively. Namely, among various materials which are useful for depositing the high refractivity layers, TiO2 is used in most cases for its high refractivity. The large difference in refractivity between the high refractivity material TiO2 and the low refractivity material SiO2 makes it possible to reduce the number of layers in producing a dielectric coating which is capable of reflecting a laser beam of a specific wavelength.
In this connection, recently optical pickups are required to cope with not only CDs (Compact Discs) using a laser beam of 780 nm in wavelength and DVDs (Digital Versatile Discs) using a laser beam of 650 nm but also to mass storage optical discs (using the so-called blue laser of 405 nm in writing and reading data). In a case where high and low refractivity layers are formed of TiO2 and SiO2, with a large difference in refractivity as mentioned above, a dielectric multi-layer coating with optical characteristics of reflecting three discrete wavelengths can be formed by depositing a reduced number of high and low refractivity layers. However, TiO2 which absorbs light of short wavelength has a problem that it invites degradations in efficiency of luminous energy. That is, when a light beam of 405 nm is cast on a dielectric multi-layer coating consisting of alternately laminated layers of TiO2 and SiO2, part of energy of incident light is absorbed by the action TiO2 layers. Therefore, part of incident light is not reflected by the dielectric multi-layer coating, resulting in a drop in efficiency of luminous energy.
In this regard, Japanese Laid-Open Patent Application H3-12605 discloses a reflecting mirror with a dielectric multi-layer coating which is arranged to avoid the above-mentioned problem of light absorption.
In the case of Japanese Laid-Open Patent Application H3-12605, thin layers of a first group are formed on a substrate, and then thin layers of a second group are formed on the first group. The thin layers of the first group are formed by the use of a material which is absorptive of light in an ultraviolet wavelength range, while the thin layers of the second group are formed by the use of a material which is not absorptive of light in the ultraviolet wavelength range. Ultraviolet light rays incident on the reflecting mirror are reflected off by the thin layers of the second group, that is to say, incident ultraviolet light rays are reflected off without being absorbed by the mirror. On the other hand, as described in Japanese Laid-Open Patent Application H3-12605, the thin layers of the first group are formed of a material which is absorptive of light in an ultraviolet wavelength range. This is because the use of a material which is absorptive of light in an ultraviolet wavelength range is effective for broadening a reflecting band width which appears in an ultraviolet wavelength range.
In the case of a reflecting mirror with a dielectric multi-layer coating which is formed by alternately laminating a high refractivity layer and a low refractivity layer one after another, lamination of a great number of layers is necessary in order to impart optical characteristics for reflection of a plural number of discrete wavelengths. In Japanese Laid-Open Patent Application H3-12605 mentioned above, the layers are divided into a first group and a second group but as a whole the number of the layers is not reduced.